Make sure you are not running IE in compatibility mode. Also uninstall all previous installations of the player in your browser. Then install the player.

For installing the Mac plug-in follow the instructions on the screen.
Here with some additional info:

1. Download (save) the disk image file 2020PluginInstaller.dmg
2. Mount the disk image (open/run the downloaded file) if not mounted automatically
3. Find the 2020PluginInstaller window (might be behind the window on top)
4. Drag the lego piece to the folder (if you do not have admin privileges you will be prompted to authenticate)
5. Restart the browser (it is not enough to close the browser window, you need to restart the browser application)
6. Access the same link and you will now see the Start button and be able to start planning your room!

Remember, Rome wasn't built in a day.
Be sure to give yourself ample time to design your kitchen perfectly.When you have finished:
Print out your drawings and product list at home or save your plan to the IKEA server. Then talk to a member of store staff to make your dream kitchen a reality.